As an example of the drive power transmission control apparatus for vehicles, there are types in which the drive power transmission amount is controlled by duty-controlling an electromagnet in an “electromagnetic clutch” as disclosed in Japanese Utility Model No. HEI6-16731 or “drive power transmission control apparatus” as disclosed in an applied specification of JP No. 2001-003937 A. In such a drive power transmission control apparatus, magnetic force for attracting an armature to the electromagnet is generated by supplying excitation current duty-controlled to an electromagnetic coil of the electromagnet, so that the armature is attracted to the side of a friction clutch. Consequently, the clutch is engaged with a pressure corresponding to the magnitude of the magnetic force so as to transmit the drive power. That is, torque to be transmitted to the front wheels and rear wheels of a vehicle is divided depending on a current value of the excitation current to be supplied to the electromagnetic coil of the electromagnet.
The excitation current to be supplied to the electromagnetic coil is detected by a current detection circuit for detecting that excitation current and inputted to the electronic control unit (ECU) of the drive power transmission control apparatus as a current detection signal. As a result, a control loop in which the input of the current detection signal is negative fed-back is constructed. Thus, the excitation current following a current instruction value determined by the ECU can be supplied to the electromagnetic coil of the electromagnet.
The current detection signal to be outputted from the current detection circuit is generally outputted through an operational amplifier after it is converted to a voltage signal. A configuration in which the voltage from an amplifying circuit is outputted as an offset voltage when no current is supplied to the electromagnetic coil of the electromagnet, that is, the value of the excitation current is 0 A (zero ampere) is often adopted. On the other hand, the output voltage by the current detection signal when the excitation current is 0 A is always deflected due to changes in the ambient temperature of the current detection circuit, deviation of the electric characteristic of component part and the like.
Thus, when the value of the excitation current is 0 A, so-called zero point adjustment in which the output voltage of the current detection circuit at that time is set up as an offset voltage needs to be carried out. Conventionally, a period in which no current is supplied to the electromagnetic coil of the electromagnet at the time of startup is provided and the offset adjustment control processing which performs the zero point adjustment in this while is carried out by the ECU which is the drive power transmission control apparatus.
However, in the above-described offset adjustment processing, most semiconductor components, resistors, capacitors and the like of an operational amplifier and the like constituting the current detection circuit have a particular temperature characteristic that their amplification factor or impedance is changed due to changes in the temperature. For the reason, so-called temperature drift that the voltage of a detection signal is changed depending on changes in the temperature of the current detection circuit may occur. Consequently, there is generated a deviation (Vof1–Vof2) in the offset voltage value between the offset voltage value Vof1 (for example, ambient temperature of 25° C.) at the time of offset adjustment processing executed just after the startup and the offset voltage value Vof2 (for example, ambient temperature of 80° C.) just after a temperature rise. Because the deviation of the offset voltage value affects the voltage value of a detection signal from the current detection circuit directly, an error occurs in distribution of torque to be transmitted to the front wheel and rear wheel of the vehicle because of current control containing such an error. As a result, there exists such a problem that a scheduled torque distribution cannot be controlled precisely.
It may be considered depending on people that the not so remarkable error in the current detection signal generated due to such a temperature drift hardly affects a feeling of passengers in the vehicle even if there is generated the error in the distribution of the torque between the front wheel and rear wheel of the vehicle. However, the improvement of the control accuracy in the vehicle drive power transmission control apparatus is an indispensable matter because the improvement of the control accuracy in the vehicle drive power transmission control apparatus relating to such a factor contributes to suppression of the deviation in the entire motion control system.
The present invention has been achieved to solve the above-described problem and an object of the present invention is to provide an offset adjustment method for a current detection signal in the drive power transmission control apparatus capable of controlling the torque distribution precisely.